This invention relates to a turbofan engine, and more particularly, the invention relates to managing the maximum speed of a low pressure turbine.
A typical turbofan engine includes low and high spools. The low spool is coupled to a turbofan and typically supports a low pressure turbine and compressor. The high spool typically supports a high pressure turbine and compressor. The spools, turbines and compressors are housed in a core nacelle. The turbofan is arranged upstream from the core nacelle. A fan nacelle surrounds the turbofan and core nacelle to provide a bypass flow path having a nozzle area through which bypass flow from the turbofan exits.
The turbines are designed to accommodate a maximum operating speed plus a margin. The engine becomes heavier and more costly as the maximum speed increases for a given turbine design. For example, the turbofan, low pressure turbine, low spool and fan containment system must be designed more robustly for a higher low pressure turbine maximum speed. Current commercial turbofan engines use fixed area nozzles that limit the ability to operate the engine to a fixed characteristic, for example maximum low pressure turbine speed. As a result, the engine must be designed for the condition requiring the maximum turbine speed during a flight envelope, even though the condition may rarely occur during typical aircraft usage.
Low bypass ratio turbofan engines are used in military fighter aircraft. These turbofan engines use variable area nozzles to balance thrust requirements, maximum rotor speed and fan stability requirements. Military fighter engines encounter maximum rotor speeds at different flight conditions compared to commercial engines. For example, military aircraft fly at maximum speed exceeding the speed of sound whereas the maximum flight speed of commercial aircraft is below the speed of sound, for example around Mach 0.8. The thrust requirement of commercial and military fighter aircraft are also significantly different.
What is needed is a turbofan engine capable of providing the needed thrust throughout the flight envelope without increasing the maximum speed of the low pressure turbine.